The present invention relates, in general, to respiration systems, and more particularly to such a system which inherently provides a safe, reliable operation and which is easily controlled, highly flexible, and capable of a great variety of operational modes which permit a wide choice of functions for maximum effectiveness in use.
This application is also related to copending application Ser. No. 402,679, of Theodore B. Eyrick and Allen C. Brown, filed on even date herewith and entitled "Breathing Gas Delivery Cylinder for Respirators", now U.S. Pat. No. 3,932,066 and application Ser. No. 402,678, of Theodore B. Eyrick and Neil R. Hattes, filed on even date herewith and entitled "Respiration Ratemeter", now U.S. Pat. No. 3,887,795, the disclosures of which are hereby incorporated herein.
Numerous types of respiration devices and systems have been developed in the prior art, and have at one time or another been on the market. Such systems have all generally had characteristics and features which met special needs or which overcame specific problems, and thus were of value to the art in specific circumstances. But, unfortunately, many of the prior art systems had flaws which made them inappropriate for use in some cases, or which made them unreliable, difficult to control, or inaccurate, and thus the search has continued for an improved respirator which would overcome such difficulties.
The basis types of respirators are well known, having been in clinical use for many years and often discussed, both as to advantages and shortcomings, in the technical literature. Some of the respirators now in use are described by W. W. Mushin, L. Rendell-Baker, P. W. Thompson and W. W. Mapleson in their book Automatic Ventilation of the Lungs, 2nd Ed., F. A. Davis Co., Philadelphia, Pa. (1969), and by W. T. Heironimus in Mechanical Artificial Ventilation, C. C. Thomas, Springfield, Ill. (1967), as well as in other publications.
Broadly speaking, respirator devices can be classified by the parameter which controls the cycling of the machine: pressure, time, or volume of gas, and many currently available machines can be so classified. A pressure-cycling machine allows air to be delivered to a patient until a preset pressure is reached, at which time the control system closes the valve controlling air flow. A disadvantage of the pressure-cycled machine is that it will deliver varying quantities (tidal volumes) of air when the pulmonary back pressure or the compliance of the system changes. This requires that the ventilation be very closely monitored to insure that the gas level of the blood does not fall outside desired limits. Generally, too, such systems have relatively low pressure capabilities and present problems in controlling the amount of oxygen delivered to the patient.
A time-cycled machine sets the time for inspiration and expiration, so that the volume of air actually delivered becomes a function of flow rate. Since the flow is limited by plumonary resistance, changes in this factor leads to variations in the volume delivered. Most such systems also have relatively low peak pressures.
Volume-controlled machines provide a relatively constant tidal volume delivery, except, under very high pulmonary resistance conditions, where losses due to the compressibility of the gas becomes a factor. These devices generally have high pressure and flow capability, but may be limited in the particular features made available in a given machine. Although volume-controlled machines provide the advantage of delivering a desired quantity of air on each cycle, the utilization of this concept has not been without problems in prior art machines. For example, in a volume-controlled machine it is essential that the measurement of volume be very accurate to insure that the machine functions properly. However, prior machines have not been reliable in this regard, and because of compliance in the machine itself, difficulties in obtaining an accurate determination of the location of a movable piston or like air drive mechanism, and sluggish mechanical or electrical control systems, it has not been possible to provide a volume system that would repeatedly provide a selected quantity of air for a patient.
Another difficulty encountered with volume-controlled systems is the likelihood of encountering excessive pressures, and such systems thus can produce a considerable safety problem. Where a gas delivery machine is arranged to sense and be controlled by the volume of gas being delivered, high pressure can appear, and it then becomes necessary to provide often complex sensing and control systems to guard against injuring the patient. However, the fact that dangerous pressures can be produced is a safety hazard in itself, for failure of the control system can result in damage. For example, pressure relief valves are commonly used in such systems to regulate the maximum pressure that can exist; but the operational characteristics of such valves can change over a period of time, and if one should fail at the wrong time, a patient could be injured.
In a volume system, it is common to provide a bellows or other air container which will receive the gas to be delivered to the patient. The container is then compressed mechanically or pneumatically to discharge the air to the patient when the inhalation portion of a breathing cycle is reached. In such systems it is customary to fill the container to its maximum volume prior to the inhalation, and to then terminate the discharge when the desired volume has been delivered. A real danger exists with this arrangement, however, for if the control system should fail during the discharge stroke, the full volume of air the machine is capable of delivering can be discharged into the patient's lungs, and can do irreparable harm.
A volume controlled system which relies on sensing the delivered volume and controlling the end point of the delivery stroke may also be inaccurate because of the overshoot that will occur after the stop signal has been given. Inertia within the mechanical delivery apparatus and lost motion in the controls may result in the mechanism moving past the desired stop point, and this can produce an inaccuracy in the delivery of the breathing gas.
In general, then, it can be said that although prior respirator or ventilator devices have been satisfactory, nevertheless, they have been too inflexible and have not been capable of safely meeting the needs of various patients in various circumstances. A patient whose breathing must be completely controlled, for example, requires different machine characteristics than a patient whose breathing is merely being machine assisted. Often a patient whose breathing has been machine controlled for an extended period becomes dependent upon the machine, and cannot breathe without it. Such a patient must be gradually withdrawn from the machine, and thus flexibility in operation to allow shifting from machine to patient controlled breathing is high desirable. A machine that does not have flexibility and reliability, then, cannot meet the varying needs of patients.